In the practice of magnetic resonance phenomena, the application of rf radiation to a sample and the pick up of resulting resonant signal is accomplished in a structure surrounding the sample which may be a helical coil, saddle coil, resonant cavity, or a bird cage resonator. The latter structure is the object of the present work, wherein it is desired to obtain resonant frequencies in a bird cage type structure to facilitate studies of diverse chemical constituents and/or spatial distributions of such constituents which may be appropriate to the sample. A bird cage coil is frequently described as a ladder circuit, which closes on itself, wherein the current flow down the coil is distributed sinusoidally around it. It is often asserted that the bird cage coil is essentially a transmission line. As a tuned rf circuit, it is employed in nuclear magnetic resonance apparatus for either or both of the functions of rf excitation and signal detection.
The bird cage coil differs in essential manner from saddle coils, helices and like structures in that phase shifts are employed to provide the proper current distribution. For the bird cage coil, there is a requirement that the phase shift be discretely distributed around the circumference of the coil from zero to 2.pi. (or 2.pi. k, where the mode number, k, is an integer). The phase shift of each element is quite frequency dependent and as a consequence, the bird cage coil is tuned at a discrete frequency to achieve the desired phase shift constraint. It is desirable to achieve a quadrature driven bird cage coil in order to maximize power efficiency upon transmission and signal-to-noise ratio during signal reception.
The bird cage coil is particularly well suited to large volume samples as are routinely encountered with apparatus for medical imaging and in vivo analytic spectroscopy. Bird cage coils are discussed by Hayes et al, J. Mag. Res., vol. 63, pp. 622-628 (1985); Tropp, J. Mag. Res., vol. 82, pp.51-62 (1989); Pascone, et al, Mag. Res. Imaging, vol. 9, pp395-408 (1991); Joseph and Lu, IEEE Tr. Med. Imaging, vol. 8 pp.386-394 (1989); Leifer, J. Mag. Res., vol. 124, pp51-60 (1997).
The bird cage structure may be regarded as a periodic structure which closes on itself. Periodic elements of the structure produce phase shifts which must aggregate to some integer multiple of 2.pi. when summed over the closed loop. Geometrically, the resonator has cylindrical symmetry and it is desired that the if current in the axial direction along the periphery of the structure be proportional to sin k.theta. and/or cosk.theta. where .theta. is the azimuthal angle about the cylindrical axis.
In the prior art, a balanced bird cage coil is driven (or signal derived) by coupling the signal ground to the midpoint of one (reactive) leg and the signal lead is suitably coupled to one end of the leg.
This prior art includes a birdcage coil having N=4n legs (where n is an integer) operated in balanced quadrature mode by coupling the signal ground of the Q signal to the electrical midpoint of a first leg and the Q signal lead to the annulus proximate to the end of that first leg while the I signal ground is coupled to the electrical midpoint of a second leg and the I signal lead is coupled to the annulus proximate to the end of the second lead, where the first and second legs are relatively phase shifted by .pi./2. This prior art is described in U.S. Ser. No. 08/768,037, incorporated herein by reference.
Although the occurrence of parasitic currents on the outer conductor of the coaxial cable driving the prior art balanced birdcage coil is reduced, there remains an inherent interaction of the balanced coil with the coax outer conductor by capacitive and radiative coupling.